An aerosol is a colloidal system consisting of very finely divided liquid or solid particles dispersed in and surrounded by gas. Aerosol dispensers typically include a pressurized container made of metal or glass which is provided with a discharge valve. The container is filled with a product to be sprayed with the propellant gas under pressure. The product that is to be dispersed as an aerosol may have the liquified propellant mixed with it in the form of a solution or the propellant may be present as a separate aqueous phase in the dispenser.
A wide variety of compositions would be highly useful if they could be dispersed from an aerosol container. However, many compositions have a relatively short shelf life due to polymerization or other reaction between individual chemicals of the composition. Also, some compositions contain chemicals which tend to destroy the gasket material of the valve mechanism which greatly limits the shelf life of aerosol containers filled with such compositions. Proper gasket selection may decrease stability problems in some cases but may present an added expense due to the alternative gasket material employed.
Some inventors have sought to avoid these probems by utilizing a aerosol can divided into two compartments. In Curry et al, U.S. Pat. No. 3,966,087 the product is dispensed from an aerosol can having two compartments connected by a gas valve. One compartment contains the product and the other contains propellant gas. However, in the device of that patent some of the material from the lower compartment is always present in the upper compartment.
Another mechanism to hold the reactants in common storage under non-reacting conditions within an aerosol container is described in U.S. Pat. No. 3,791,980. In that patent one of the ingredients is encapsulated in the form of drops in small protective shells. The shell prevents reaction between the reactants until ready for use at which time the reactants, including the coated droplets, are discharged through a single valve. The reactions actually mix outside of the aerosol container. This procedure is of course highly limited since it requires the formation of extremely small drops which must then be coated. The coated droplets must be kept separate from each other so as not to clump together to form a large aggregate.